The present invention relates generally to medical imaging systems and, more particularly, to a radio frequency (RF) receiver coil array for a magnetic resonance (MR) imaging system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of nuclear magnetic resonance (NMR) signals are received by a RF coil array and subsequently digitized and processed to reconstruct the image using one of many well known reconstruction techniques. With respect to the RF coil array, MR systems often include a dedicated receiver coil array that is integrated into a patient table or formed as a separate surface coil, with the receiver coil array comprising a two-dimensional array formed from a plurality of coils.
To minimize inter-element coupling between the individual receiver coils, RF receiver coil arrays are typically overlapped in such a way that each coil element exhibits negligible mutual inductance with nearest neighboring coil elements. In one known arrangement, shown in FIG. 1, an array 102 of coil elements 104 is arranged in a rectangular lattice. In such an arrangement, coupling each coil element 104 and those coil elements positioned to the left and right, and those coil elements positioned above and below, is minimized. However, in the arrangement of FIG. 1, each coil element 102 has substantial coupling with neighboring coil elements diagonal thereto, thus providing a non-optimal arrangement.
To overcome this drawback, overlapped arrays, such as the array 106 of FIG. 2, typically have alternate columns 108 (or rows) of coil elements 110 staggered by one half the element width. The overlap between coil elements 110 is such that a hexagonal lattice is created, where each coil element 110 is optimally overlapped to produce zero or negligible coupling with six neighboring coil elements. One drawback of this arrangement is that the staggering between alternate columns/rows can result in holes 112 in the coil array around the edges of the array. In each of these holes 112, signal-to-noise ratio (SNR) is reduced, resulting in degraded image quality in these areas in a reconstructed image.
It would therefore be desirable to have a coil array in which empty areas devoid of coil coverage are eliminated. It would also be desirable that such a coil array have minimal coupling between nearest-neighboring coil elements.